Memory bandwidth is a crucial resource in computing systems. Current CMP/SMT processors have a significant number of cores and they can run many threads concurrently. This large thread count adds high pressure to the memory bus, which demands high bandwidth to service memory requests from the cores. Hardware data prefetching is a well-known technique for hiding memory latency. Due to its speculative nature, however, in some situations prefetching does not effectively work, wasting memory bandwidth and polluting the caches. Data prefetching efficiency depends on the prefetching algorithm. It also depends on the characteristics of the applications running on the system. In this paper we propose an online bandwidth shifting mechanism that dynamically assigns bandwidth to applications according to their prefetch efficiency. This mechanism maximizes the utilization of memory bandwidth, thereby improving system performance and/or reducing memory power consumption. To the best of our knowledge, this solution is the first to not require hardware support. We evaluate the benefits of using our bandwidth shifting mechanism on a real system - the IBM POWER7. We obtain speedups in the order of 10-20% (in one instance, speedup exceeds 1.6X). Our mechanism does not generate a significant degree of unfairness among the applications. In many cases individual thread performance increases by 10-35%, while virtually no thread experiences a slowdown larger than 5%.

This s work has been partially sponsored by Defense Advanced Research Projects Agency (DARPA), Microsystems Technology Office (MTO), under contract no. HR0011-13-C- 0022. The views expressed are those of the authors and do not reflect the official policy or position of the Department of Defense or the U.S. Government. This document is: Approved for Public Release, Distribution Unlimited. This work has also received funding from: the Spanish Ministry of Science and Innovation under grant TIN2012-34557 and the HiPEAC Network of Excellence; and the European Research Council under the European Unions 7th FP (FP/2007- 2013) / ERC GA n. 321253. Additional support was received from a joint study agreement between IBM and BSC (number W1361154).

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